Maxillary palps

In addition to the sensilla styloconica, lepidopterous larvae possess gustatory sensilla on the maxillary palps. Eight basiconic sensilla are located on top of each palpus (see Figure 1). Five of them possess a terminal pore, and for that reason these sensilla might be considered as contact chemoreceptors. The remaining three show numerous small perforations all over the cuticle, which indicates an olfactory function (.8). The response spectra of these sensilla are, however, still obscure. 

A number of chemo- and mechanoreceptors participate in the male behaviors. The female contact pheromone is detected by chemosensilla on the antennae and labial and maxillary palps (Ramaswamy and Gupta, 1981). The number of these sensilla increases dramatically during the metamorphic molt, and much more so in males than in females. Unfortunately, no electrophysiological recordings have been conducted, and the specific sensillum type that responds to the contact pheromone... 

Ramaswamy, S. B. and Gupta, A. P. (1981). Sensilla of the antennae and the labial and maxillary palps of Blattella germanica (L.) (Dictyoptera Blattellidae) their classification and distribution. Journal of Morphology 168 269-279. 

Grant A. and O Connell R. (1996) Electrophysiological responses from receptor neurons in mosquito maxillary palp sensilla. In Ciba Found. Symp., eds G. Bock and G. Cardew. Wiley, Chichester. 

Potential PBPs (OS-E and OS-F) U02545, U02546 identified on the third antennal segment of adult males (McKenna etal. 1994) PBP-related proteins found in different subsets of olfactory sensilla (Pikielny etal. 1994) First insect olfactory receptors (of unknown odorant selectivity) identified on the maxillary palp (Clyne etal. 1999)... 

In S. gregaria, immunocytochemical experiments showed selective CSP labeling of the outer lymph in diverse chemosensory organs, including contact sensilla of tarsi, maxillary palps, and antennae. However, in antennae, only sensilla chaetica were labeled with no labeling observed in olfactory or coeloconic sensilla (Angeli et al., 1999), suggesting a role for CSPs in contact chemoreception in Orthoptera. 

Figure 19.1 Or85e is selectively expressed in subsets of maxillary palp neurons. In situ hybridization with an antisense Or85e probe reveals no staining in antenna (left) or brain (center). Nine cells stain in this section of the maxillary palp (right). The relative positions of the two olfactory sensory organs on the head of the adult fly are indicated with white dashed lines.

OR name Previous names Entres SwissProt Map Antenna Maxillary palp Not detected... 

The second Drosophila appendage that bears olfactory sensilla is the maxillary palp (henceforth simply called palp). The ca. 60 olfactory sensilla on the dorsal and lateral sides of the palp are sw sensilla of the B type. They are structurally similar to antennal TB sensilla and contain two ORNs. In addition to olfactory sensilla the palp also bears ca. 20 large mechanosensory bristles. 

Moreover, the pbl A neuron on the maxillary palps also responds to these levels of Zll-18Ac, but is clearly much more sensitive to other esters (Figure 23.2B). Stocker and Gendre (1989) did demonstrate a role for palpal sensilla in male courtship inhibition, so if Zll-18Ac plays a pheromonal role, at least two ORN classes encode it. This compound has also been suggested to function as an aggregation pheromone (Bartelt el al., 1985). 

Ayer R. and Carlson J. R. (1992) Olfactory physiology in the Drosophila antenna and maxillary palp at]6 distinguishes two classes of odorant pathways. J. Neurobiol. 23, 965-982. 

Riesgo-Escovar J. R., Piekos W. B. and Carlson J. R. (1997b) The maxillary palp of Drosophila ultrastructural and physiology depends on the lozenge gene. J. Comp. Physiol. A 180, 143-150. 

Singh R. N. and Nayak S. V. (1985) Fine structure and primary sensory projections of sensilla on the maxillary palp of Drosophila melanogaster Meigen (Diptera Drosphilidae). Int. J. Insect Morphol. Embryol. 14, 291-306. 

Grant A. J., Wighton B. E., Aghajanian J. G. and O Connel R. J. (1995) Electrophysiological responses of receptor neurons in mosquito maxillary palp sensilla to carbon dioxide. 

Crude efforts were made to assess the effect of the removal of the maxillary palps and maxillae, both accessory mouth parts with many receptors assumed to be connected with the senses of taste or smell. The results were inconclusive, since insects so operated on refused to feed on either treated or untreated leaves, although this could also be interpreted to indicate that 24,055 had the same effect as removal of the sensory receptors—that is, it inhibited the response of the receptors so that the insect failed to recognize the treated material as food. 

The diverse morphology of the olfactory organ among insect species provides a basis upon which classification has been based. Basically, the olfactory organ in the adult insect comprises two components on the head the antenna and the maxillary palp (Fig. la). Numerous sensilla cover the surface of the antennae and prevent direct contact of ORNs with the external environment. Each sensillum is filled with a potassium- and protein-rich fluid called sensillum lymph and houses one to four ORN dendrites. The small pits on the cuticle surfaces of sensilla allow contact of the ORN dendrite with volatile odorants that dissolve in the lymph. In Drosophila, the third segment of the antenna and of the maxillary palp possess approximately 1,200 and 120 ORNs, respectively. 

Fig. 1 Olfactory organ of insects. Insects have two types of olfactory organ antenna and maxillary palp. Antennae and maxillary palps both bear numerous sensilla. An individual sensillum houses between one and four dendrites of olfactory receptor neurons, a Morphology of the olfactory organ of the adult silkmoth. b Morphology of the olfactory organ of silkmoth larvae...

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